It has Colt logo and name under that is "AR-15 A2 cal.223 sporter II".It has 20" 1in7 twist barrel. Forward assist. A1 style sites. .312 diameter front take down pin. I suspect the upper is different from original, because of slight color change. It functions flawlessly.

Sounds like an R6500. While anything is possible, the slight color difference between the upper and lower receivers does not necessarily indicate that parts have been swapped around. Given the limited amount of information that you have provided, I would say $1100-$1200 would be a good ballpark estimate.